This invention relates to valves, and more particularly to a block valve for use in a hydraulic control circuit for the operation of a downhole safety valve.
In the course of operation of an oil or gas well, it is sometimes desirable and necessary to provide a shut-off capability for permitting the conduct of maintenance operations or for preventing leakage of fuel to the outside environment in the event of an accident. A device which is commonly employed to provide this capability is a surface controlled subsurface safety valve otherwise known as a downhole safety valve (DHSV) which is installed in the production tubing of the well at a specified depth below the wellhead. Such valves are of the "fail-close" design and are customarily operated by a hydraulic actuator located in the well and controlled by a safety controls system located at the surface.
The safety system which is adapted to respond to an excessive or deficient well flow pressure to close the DHSV and shut-off the flow of well fluids from the well, typically includes shut-off relay valves in the control line which, if a leak should occur, will shut-off the supply of pressurized fluid to the downhole actuator and permit the DHSV to close. It is important, however, in shutting off the supply of pressurized fluid to the actuator that the system will permit a sufficient bleeding of the pressurized fluid from the actuator so that the DHSV does not remain open or partially open. Accordingly, the relay valves must not shut-off the control line supply to the downhole actuator until an appropriate time delay so as to permit the actuator for the DHSV to bleed its pressure into a discharge reservoir during the delay. In addition, after the DHSV is closed, the shut-off valves must completely block communication between the DHSV actuator and the reservoir otherwise an equipment failure, such as a packing failure in the DHSV, might cause the well to bleed back into the discharge reservoir with potentially disastrous results.
The use of a fluid controlled delay circuit in controlling the flow of pressurized fluid from a downhole actuator is described in U.S. Pat. No. 4,215,746 and a hydraulic circuit for opening and closing a downhole safety valve which utilizes block valves controlled to allow complete closure of an associated DHSV is disclosed in U.S. Pat. No. 4,193,449. However, the block valves in these systems require many components and considerable piping with attendant disadvantages of reduced reliability, high manufacturing costs, and large space requirements.
It is an object therefore to provide a new and improved block valve for use in a hydraulic system for controlling a DHSV which will provide for a positive block in the control line from the DHSV to the surface system and permit enhanced reliability with a fewer number of system components.
It is another object to provide a block valve having an inlet port, an outlet port and a bleed port and a pair of independently operable valve plunger elements, each of which is movable in response to a pressurized pilot fluid supply for controlling communication between said ports whereby the valve elements are adapted to be moved simultaneously to second operating positions to effect communication between the inlet and outlet ports while blocking communication between these ports and the bleed port and upon removal of pilot fluid pressure to be returned to first operating positions in sequence to shut-off fluid communication between the inlet port and the outlet port and establish communication of the outlet port with the bleed port for a limited period of time prior to blocking of fluid communication between all of said ports by the valve elements.